Science —

It’s (just barely, sort of) alive!

Below the ocean floor, microbes are surviving on 100,000 oxygen molecules a day.

For something we are all familiar with, life tends to be a remarkably difficult thing to define. Most definitions, however, include the ability to reproduce. Which means that a microbial community that has been discovered on the floor of the Pacific Ocean stretches the definition of what it means to be alive.

Bacteria are detected in the sediments down to the level of at least 20m, and are probably present (if rare) below that. But based on oxygen consumption, the cells are operating at a metabolism that appears to be right at the minimum energy flux needed to simply keep their cellular components operational. With no energy to spare, it's possible that these cells are not even able to reproduce.

The discovery of thriving ecosystems at deep-ocean hydrothermal vents helped revolutionize how we view life. Unlike familiar ecosystems on the surface, these organisms weren't ultimately dependent upon sunlight to power the base of the food chain, instead relying on chemical energy provided by the Earth's internal heat. Since then, other communities have been found that rely on unusual sources of energy. Deep in a mine, organisms appear to rely on radioactive decay to provide a source of hydrogen. Under an Antarctic ice sheet, another community grabs its energy by oxidizing iron exposed by the glaciers.

These organisms have provided us with insights about the broad range of energy sources that can be used to support life. But they've also helped tell us about just how little energy life needs to squeak by. The bacteria found in the South African mine, for example, get so little energy that they're probably able to divide only once every few hundred years. In contrast, a well-fed E. coli can divide every 20 minutes.

The newly described bacterial community may make those look positively energetic.

They were spotted by a research cruise that was taking sediment cores across the equatorial Pacific before turning north and sampling up past Hawaii. This path took it into the North Pacific Gyre, an area that has been made famous because circulating currents tend to trap a lot of our trash there. But biologically speaking, it was also an area of low biological activity. The surface waters are relatively nutrient-starved, and have a primary productivity that is nearly an order of magnitude lower than the open ocean nearby.

On the ocean floor, this seems to have produced a completely different ecosystem than many other areas of the globe. Generally, the sediment that falls to the ocean floor is rich in organic compounds created by the life above. These get rapidly digested by the microbes above, which burn through the sediments oxygen at a rapid clip while doing so. As a result, just a short distance from the surface, the oxygen gets very sparse, and anaerobic microbes take over.

The sediment in the gyre is nothing like that. It takes roughly a thousand years for just a millimeter to accumulate, and bacterial activity is so low that there's oxygen down to at least 30 meters—at which point, the sediment was probably deposited about 86 million years ago. If big numbers are a problem, think of it this way: that's back when dinosaurs dominated terrestrial ecosystems.

The authors searched their sediments for bacteria, and were able to detect them down to at least 20m, at which point there were only about 1,000 cells in every cubic centimeter of muck. There may be more below that, but they were below the authors' ability to detect them. Based on the density of the cells present and the rate at which oxygen decreased with depth, the authors were able to calculate how much oxygen was being used to keep these cells alive.

The results were pretty eye-popping. Thirty meters below the ocean floor, an entire liter of sediment would only burn through a nanomole of oxygen—in an entire year. At that rate, a single cell would be using just 100,000 oxygen molecules a day to stay alive. That's below the rate described for the slowest-growing cultures we've managed to maintain in a lab and probably closer to the oxygen use of bacteria in a quiescent, stationary phase of growth. "These microbial communities may be living at the minimum energy flux needed for prokaryotic cells to subsist and that the total available energy flux ultimately controls the microbial community size in the deep biosphere," the authors conclude.

In other words, they've only been receiving enough energy to do maintenance, and probably not enough to actually divide. Which stretches most definitions we have of what it means to be alive.

The biggest question lingering over the story is what's actually going on once oxygen use reaches its steady state level in these sediments. Given the difficulty of detecting cells, the authors haven't clearly demonstrated that a non-biological process doesn't take over below a certain depth. Gently adding some nutrients and a bit more oxygen might allow something to grow out of the sample, providing a clearer indication that there really is something alive down there.

And, if something does grow out, then we could actually subject it to DNA sequencing and figure out what exactly can live under such extraordinarily limiting conditions.

For comparison, say that in one breath we breath about a liter of air. At 25C a mole of air would occupy 24.5 liters. Say air is 1/5th oxygen, that we breath about 12 times/minute# and that we use about 1/4 of the oxygen in the air##, we get: (1/24.5) * (1/5) * 1/4 * 12 * 60 * 24 * 365 * 10^^9 ~= 13 trillion nano moles of oxygen used by a human in a year.

Or, humans use about 13,000 billion billion times as much oxygen as these mud bugs in a given unit of time.

38 Reader Comments

The obvious questions are "if they are not able to breed, how are there so many of them at that depth? How did they get there?" I personally believe that we will eventually find life on other planets but it's going to be something similar to this. I really don't think we will ever find sentient, much less human-like, life.

The obvious questions are "if they are not able to breed, how are there so many of them at that depth? How did they get there?"

Likely by getting compacted under layers of failing sediment over time. So near the surface they are able to divide since they have enough energy available. This builds a population that then over times get sediment on top of it reducing the energy available more and more to the point that they get in this state.

...of course it is equally possible (or a mix) that some cell division takes place for a lucky few every now and then, given the likely time scales this would build a population and maintain it.

K1LLTACULAR wrote:

I personally believe that we will eventually find life on other planets but it's going to be something similar to this. I really don't think we will ever find sentient, much less human-like, life.

I personally cannot understand argument for the later part of your belief (unless you are just stating how hard it is to get between star systems on our lifetime scales). It is likely – given the numbers – that life exists on a lot of planets likely dominated by simple life like this but some number would have far more advanced life, likely a few exceeding our own planet. Very very little reason to believe our planet is so radically unique.

I'm confused about this idea that the definition of "alive" must include "ability to reproduce," at least as applied to individual life-forms. I can understand it as a species-level biological indicator, but used like this? Does that mean that if I am ugly and unable to get a girlfriend, I am possibly dead? What if I happen to live in an area with no females? My status as life-form is questionable? What if I am simply infertile, or too lazy to reproduce?

I'm confused about this idea that the definition of "alive" must include "ability to reproduce," at least as applied to individual life-forms. I can understand it as a species-level biological indicator, but used like this? Does that mean that if I am ugly and unable to get a girlfriend, I am possibly dead? What if I happen to live in an area with no females? My status as life-form is questionable? What if I am simply infertile, or too lazy to reproduce?

Exactly, if "ability to reproduce" applies to individuals instead of to the species as a whole then women who have been through menopause are dead.

There are also living organisms such as mules who cannot reproduce. They are a hybrid of two species, but no mule is fertile. Are mules alive?

Agreeing with pusher and sai. Further, if these microbes were cultured in an environment with higher oxygen content, would they reproduce? The proposition that to be alive requires the ability to reproduce on an individual level is too simplistic.

Once exposed to high levels of oxygen, the curious microbes began to reproduce exponentially. The microbe population soon infected one of the lab assistants working on the specimen, who began to exhibit strange neurological symptoms. Within hours, he was completely deranged, attacking coworkers and trying to eat them. The infection quickly spread down the West Coast and throughout the world.

There was nothing that could stop it. The zombie apocalypse had begun.

Further, if these microbes were cultured in an environment with higher oxygen content, would they reproduce?

That depends on whether these are obligate anaerobes or facultatively anaerobic organisms. Most likely its a complex population of both. Most obligate anaerobes would die very quickly under oxic conditions.

The proposition that to be alive requires the ability to reproduce on an individual level is too simplistic.

No, not really.

There are still huge amount of cells within this ecosystem and very little as far as predators are concerned. Even phages are likely to have little effect on population numbers in a such slowly growning community. Couple hundred years average for replication cycle doens't mean that its not just individual cells replicating under those conditions.

amireal wrote:

They still reproduce mitotically

Only Eukaryotic cells reproduce mitotically. These are bacteria ( and possibly archaea as well). They would, of course, still replicate trough cellular division.

I'm confused about this idea that the definition of "alive" must include "ability to reproduce," at least as applied to individual life-forms. I can understand it as a species-level biological indicator, but used like this? Does that mean that if I am ugly and unable to get a girlfriend, I am possibly dead? What if I happen to live in an area with no females? My status as life-form is questionable? What if I am simply infertile, or too lazy to reproduce?

They *do* have classifications such as "nerd" for such things - and even those sub-species have been known to reproduce well given the proper stimuli.

All joking aside, I think the magic word here is "ABILITY" as in, given the proper circumstances, is it possible for this species to reproduce. Not the individual - just as some of us may not be able to reproduce, some cells might not be able to either. That's an exception, not the rule.

So, if this particular strain of bacteria *can* reproduce in some way shape or form, it still can be classified as "living" or "alive". Make sense now?

Further, if these microbes were cultured in an environment with higher oxygen content, would they reproduce?

That depends on whether these are obligate anaerobes or facultatively anaerobic organisms. Most likely its a complex population of both. Most obligate anaerobes would die very quickly under oxic conditions.

The microbes in question are aerobes, as confirmed by a quick look at the referenced study. This is why oxygen availability is the limiting factor for consideration.

Presumably exposure to higher levels of oxygen would allow cellular division, but there's only one way to find out for sure... It would be really damned interesting if these microbes had jettisoned the ability to reproduce.

For comparison, say that in one breath we breath about a liter of air. At 25C a mole of air would occupy 24.5 liters. Say air is 1/5th oxygen, that we breath about 12 times/minute# and that we use about 1/4 of the oxygen in the air##, we get: (1/24.5) * (1/5) * 1/4 * 12 * 60 * 24 * 365 * 10^^9 ~= 13 trillion nano moles of oxygen used by a human in a year.

Or, humans use about 13,000 billion billion times as much oxygen as these mud bugs in a given unit of time.

The microbes in question are aerobes, as confirmed by a quick look at the referenced study. This is why oxygen availability is the limiting factor for consideration.

Presumably exposure to higher levels of oxygen would allow cellular division, but there's only one way to find out for sure... It would be really damned interesting if these microbes had jettisoned the ability to reproduce.

I suspect the limiting factor is lack of an available carbon source. The anaerobic lifestyle isn't necessarily problematic for prokaroytes.

For comparison, say that in one breath we breath about a liter of air. At 25C a mole of air would occupy 24.5 liters. Say air is 1/5th oxygen, that we breath about 12 times/minute# and that we use about 1/4 of the oxygen in the air##, we get: (1/24.5) * (1/5) * 1/4 * 12 * 60 * 24 * 365 * 10^^9 ~= 13 trillion nano moles of oxygen used by a human in a year.

Or, humans use about 13,000 billion billion times as much oxygen as these mud bugs in a given unit of time.

I think it's more interesting if you compare the average rate used by a human cell with what these bacteria are using. The estimate for human cell count varies widely (I was able to find values ranging from 10^13 to 10^15 on the first page of Google results); I'm taking a midpointish value of 1.3 * 10^14 to make the math easy. That puts human cells as using about 100,000,000x more O2.

I'm confused about this idea that the definition of "alive" must include "ability to reproduce," at least as applied to individual life-forms. I can understand it as a species-level biological indicator, but used like this? Does that mean that if I am ugly and unable to get a girlfriend, I am possibly dead? What if I happen to live in an area with no females? My status as life-form is questionable? What if I am simply infertile, or too lazy to reproduce?

Kind of like when one's ability to reproduce is described as "fitness". Unable to reproduce means your fitness is 0.

For comparison, say that in one breath we breath about a liter of air. At 25C a mole of air would occupy 24.5 liters. Say air is 1/5th oxygen, that we breath about 12 times/minute# and that we use about 1/4 of the oxygen in the air##, we get: (1/24.5) * (1/5) * 1/4 * 12 * 60 * 24 * 365 * 10^^9 ~= 13 trillion nano moles of oxygen used by a human in a year.

Or, humans use about 13,000 billion billion times as much oxygen as these mud bugs in a given unit of time.

The 100.000 molecules a day number is impressive, but it is even more impressive when you realize that's one molecule of oxigen per second. One per second. The whole rest of the bacteria, all other trillions of molecules in it, must be having close to zero chemical reactions during the rest of the second. That's.... amazing.

For comparison, say that in one breath we breath about a liter of air. At 25C a mole of air would occupy 24.5 liters. Say air is 1/5th oxygen, that we breath about 12 times/minute# and that we use about 1/4 of the oxygen in the air##, we get: (1/24.5) * (1/5) * 1/4 * 12 * 60 * 24 * 365 * 10^^9 ~= 13 trillion nano moles of oxygen used by a human in a year.

Or, humans use about 13,000 billion billion times as much oxygen as these mud bugs in a given unit of time.

You missed the "mole" part, which is a unit to describe the amount of molecules (in this case). 1 mole is approximately 6,022*10^23 molecules, so 1 nano mole is 10^14 molecules. 13 trillion nano moles of oxygen equals 1,3 * 10^27 oxygen molecules. Compare this to the 100 000 molecules used by this bacteria.

Also, what a fine counterargument to the stupid catastrophist notion of "let us not bring up the antarctic isolated lake life, as it can be 2 million years old and who knows what will ensue". Geological upheavals, or these sediment tests, should have exposed some of these old organisms from time to time.

I'm confused about this idea that the definition of "alive" must include "ability to reproduce," at least as applied to individual life-forms. I can understand it as a species-level biological indicator, but used like this? Does that mean that if I am ugly and unable to get a girlfriend, I am possibly dead? What if I happen to live in an area with no females? My status as life-form is questionable? What if I am simply infertile, or too lazy to reproduce?

Exactly, if "ability to reproduce" applies to individuals instead of to the species as a whole then women who have been through menopause are dead.

There are also living organisms such as mules who cannot reproduce. They are a hybrid of two species, but no mule is fertile. Are mules alive?

This is confusing characteristics of populations with characteristics of individuals. I don't think it has to do with species-level as much as what biological processes are.

NASA definitions of life (considering metabolism) or anthropic definitions of life (individual experience) are all right for identifying individual potential organisms. They can not tell you if you are looking at a live population, a biological population that is partaking in an evolutionary process. (The only generic process of life we know of.)

Evolutionary dead ends are metabolizing and sometimes experiencing organisms, if the whole population is a dead end they are nevertheless effectively extinct.

Also, I think these notions of why humans especially doesn't procreate are too simplistic and blaming those that may not have voluntarily chosen the outcome (i.e. blaming victims). Unless I am mistaken it is not rare at all, I just read somewhere that some ~ 20 % of individuals of modern societies will not procreate themselves. It appears that in most such cases related individuals propagates their genes anyway.

DanNeely wrote:

human cells as using about 100,000,000x more O2.

According to Lane's energy theory, eukaryotes handle up to 10^5 times as high energy density as prokaryotes. (Thus their ability to evolve and utilize large genomes.)

So these bacteria are at least 10^3 times less energetic than run-of-the-mill bacteria.